Discriminating the biophysical signal from human-induced effects on long-term primary production dynamics. The case of Patagonia.

The temporal trend of aboveground net primary production (ANPP) is frequently used to estimate the effect of humans on ecosystems. In water-limited ecosystems, like most grazing areas in the world, the effect of humans act upon ANPP in combination with environmental variations. Our main objective was to quantify long-term (1981-2012) changes of ANPP and discriminate the causes of these changes between environmental and human at a subcontinental scale, across vast areas of Patagonia. We estimated ANPP through a radiative model based on remote sensing data. Then, we evaluated the relation between ANPP and environmental interannual variations of two hierarchically related factors: El Niño Southern Oscillation (ENSO) through the Southern Oscillation Index (SOI), and precipitation. We described the effect of humans through the shape of the temporal trends of the residuals (RESTREND) of the environmental model and quantified human relative impact through the RESTREND: ANPP trend ratio. ANPP interannual variation was significantly explained by ENSO (through SOI) and precipitation in 65% of the study area. The SOI had a positive association with annual precipitation. The association between ANPP and annual precipitation was positive. RESTREND analysis was statistically significant in 92% of the area where the tested environmental model worked, representing 60% of the study area, and it was mostly negative. However, its magnitude, revealed through the RESTREND: ANPP trend ratio, was relatively mild. Our analysis revealed that most of ANPP trends were associated with climate and that even when human density is low, its incidence seems to be mainly negative.

Geographic Variation in Festuca rubra L. Ploidy Levels and Systemic Fungal Endophyte Frequencies.

Polyploidy and symbiotic Epichloë fungal endophytes are common and heritable characteristics that can facilitate environmental range expansion in grasses. Here we examined geographic patterns of polyploidy and the frequency of fungal endophyte colonized plants in 29 Festuca rubra L. populations from eight geographic sites across latitudes from Spain to northernmost Finland and Greenland. Ploidy seemed to be positively and negatively correlated with latitude and productivity, respectively. However, the correlations were nonlinear; 84% of the plants were hexaploids (2n = 6x = 42), and the positive correlation between ploidy level and latitude is the result of only four populations skewing the data. In the southernmost end of the gradient 86% of the plants were tetraploids (2n = 4x = 28), whereas in the northernmost end of the gradient one population had only octoploid plants (2n = 8x = 56). Endophytes were detected in 22 out of the 29 populations. Endophyte frequencies varied among geographic sites, and populations and habitats within geographic sites irrespective of ploidy, latitude or productivity. The highest overall endophyte frequencies were found in the southernmost end of the gradient, Spain, where 69% of plants harbored endophytes. In northern Finland, endophytes were detected in 30% of grasses but endophyte frequencies varied among populations from 0% to 75%, being higher in meadows compared to riverbanks. The endophytes were detected in 36%, 30% and 27% of the plants in Faroe Islands, Iceland and Switzerland, respectively. Practically all examined plants collected from southern Finland and Greenland were endophyte-free, whereas in other geographic sites endophyte frequencies were highly variable among populations. Common to all populations with high endophyte frequencies is heavy vertebrate grazing. We propose that the detected endophyte frequencies and ploidy levels mirror past distribution history of F. rubra after the last glaciation period, and local adaptations to past or prevailing selection forces such as vertebrate grazing.

Grazing intensity differentially regulates ANPP response to precipitation in North American semiarid grasslands.

Grazing intensity elicits changes in the composition of plant functional groups in both shortgrass steppe (SGS) and northern mixed-grass prairie (NMP) in North America. How these grazing intensity-induced changes control aboveground net primary production (ANPP) responses to precipitation remains a central open question, especially in light of predicted climate changes. Here, we evaluated effects of four levels (none, light, moderate, and heavy) of long-term (>30 yr) grazing intensity in SGS and NMP on: (1) ANPP; (2) precipitation-use efficiency (PUE, ANPP : precipitation); and (3) precipitation marginal response (PMR; slope of a linear regression model between ANPP and precipitation). We advance prior work by examining: (1) the consequences of a range of grazing intensities (more grazed vs. ungrazed); and (2) how grazing-induced changes in ANPP and PUE are related both to shifts in functional group composition and physiological responses within each functional group. Spring (April-June) precipitation, the primary determinant of ANPP, was only 12% higher in NMP than in SGS, yet ANPP and PUE were 25% higher. Doubling grazing intensity in SGS and nearly doubling it in NMP reduced ANPP and PUE by only 24% and 33%, respectively. Increased grazing intensity reduced C3 graminoid biomass and increased C4 grass biomass in both grasslands. Functional group shifts affected PUE through biomass reductions, as PUE was positively associated with the relative abundance of C3 species and negatively with C4 species across both grasslands. At the community level, PMR was similar between grasslands and unaffected by grazing intensity. However, PMR of C3 graminoids in SGS was eightfold higher in the ungrazed treatment than under any grazed level. In NMP, PMR of C3 graminoids was only reduced under heavy grazing intensity. Knowing the ecological consequences of grazing intensity provides valuable information for mitigation and adaptation strategies in response to predicted climate change. For example, moderate grazing (the recommended rate) in SGS would sequester the same amount of aboveground carbon as light grazing because ANPP was nearly the same. In contrast, reductions in grazing intensity in NMP from moderate to light intensity would increase the amount of aboveground carbon sequestrated by 25% because of increased ANPP.